DIYstompboxes.com

Hey everyone! This is my first post on this forum. Hoping you guys might be able to give me some pointers on why I might be experiencing the issues that I am having with my MK I build. This pedal has been kicking my butt and while I’m fairly confident that I just haven’t been able to get the proper transistor combo sorted out yet, I wanted to seek some clarification as it relates to this circuit. I’ve searched and searched for information on things like transistor voltages, target gain and leakage, etc. While there are quite a few threads out there that already broach these topics, it seems there is no “one size fits all” answer. Quite different than something like the fuzz face circuit. I’ve built the Gary Hurst version of the pedal, and while the pedal does work, I cannot seem to dial out the overly sizzly decay. It’s become quite the time consuming and frustrating process of measuring and selecting transistors, plugging them in, only to still have essentially the same result each time I play through the pedal. I understand the MKI is primitive and sizzle/gating are commonly associated with this circuit, but mine seems overly obnoxious and unpleasant. I’ve owned MKI builds that didn’t do this. The fuzz tone is not bad on mine at all, but the way notes fade out really drives me a bit nuts. It has quite a gate to it as well. Things I’d like to seek information on are the following:

What are target voltages on each legs of the transistors that I should shoot for?

Should Q1 base and Q1 emitter voltage essentially be equal? Every time I put transistors in my circuit to test, I do a voltage check first and have taken notes on different combos and values. I am noticing the voltage on Q1 base and emitter are basically equal. Is this normal or indicative of a problem?

What should I be aiming for in terms of leakage and gain on my transistors? I see what small bear shoots for in their MKI transistor combo, but have read forums where people have shared positive results with some values that differ quite substantially from one another. Everyone has their own ear but what’s more crucial here? Gain or leakage or both?

I’ve noticed if the transistors are too leaky, the pedal will squeal and make noise even if I’m not playing or touching the guitar. It seems like the window of satisfactory leakage and gain(more leakage though) is VERY small.

What’s the potential main influencer of sizzle on the note decay? It’s excessive in my build. I can see maybe a little but it’s quite intense with mine. No combo I’ve put in the pedals has really made a huge impact up to this point. I’ve measured Q2 collector voltages anywhere from 2v up to 8v, with sizzle being present in all scenarios.

Are there some transistors that just don’t work in this circuit? Some people definitely seem to believe so. I have picked through my whole stash which consists currently of:

2N404 and 2N414 (STI)
SFT353
GT402B
AC132
MP16B
3AX31C

So far the best combo I had noted down so far was:

Q1 = MP16B. Roughly 66 hfe, 104 mA leakage
Q2 = SFT353. Roughly 70 hfe, 590 mA leakage
Q3 = 2N404. Roughly 100 hfe, 62 mA leakage

Voltages with this combo:

Q1 collector: 9.18
Q1 base: 0.517
Q2 emitter: 0.47

Q2 collector: 5.81
Q2 base: 0.131
Q2 emitter: 0

Q3 collector: 9.09
Q3 base: roughly 8-10 mV
Q3 emitter: 0

I’ve seen people suggest q3 needs low to no leakage. I’ve seen other say this for q2 instead. I’ve seen others say that all positions needed higher leakage to work for them. The uncertainty with how to approach this circuit is something that I guess keeps a lot of builders away. I’d really like to get this thing dialed in and so I’m hopeful someone here can guide me!

I’ve tried higher leakage trannies in Q1 and Q2. I’ve tried high leakage in q1 and low leakage in q2. I’ve tried higher leakage in all positions. I’ve tried as many combos as my stash has allowed and am essentially not making much progress. I am considering investing in some more transistors, but wanted to put this thread out to the world first.

I appreciate any tips or suggestions anyone has! Thanks for reading!!

Hmmm…wondering if I figure the voltage drop from in to out on the 157, if I can feed one from the pots, adjust it after, then would it hold the voltage? Or…maybe look for a Schmitt trigger one, then it should hold and I can switch the after parts. Something I need to learn on, but that’s what holds its output state steady, regardless of input changes, yea?

Ok, thank you on this, I’ll riddle through a bit more…

I would triple-check and make sure the Vbias resistors (R12 and R13) are correct... if R12 was 47R instead of 47k then it would cause voltages pretty much exactly as measured. There are a few other 47R resistors in the circuit, so it would be an easy mistake to make since all but one of the color bands are the same.

I did have the correct value on R12 & 13. What about no voltage in C10? There is a trace from the intersection of R12&13 to the + pad on C10, correct? The working “B” side has voltage on + pad of C20 and that’s one of the more telling differences I can find. I even changed C10 to another cap but nothing. Am I after a red herring?

Continuity is ok, but no sound no matter the cap size

Does the circuit itself work ?

any tricks to have it remember settings? I’m guessing digital pots or something.

Well, the chips are expecting a 0-5V CV, so anything that produces that is fine. That means a DAC, digital pots, an Arduino's PWM outputs (with a bit of filtering), whatever. It's all shouting "PROCESSORS!" at me at that point though - remembering settings is a job for microprocessors and code, in whatever form you like best.